The synthesis of methanol is an important chemical engineering process. Currently, the Cu—Zn—Al catalyst is the main component of the widely-used industrial catalyst for the synthesis of methanol at a low pressure. It is generally prepared by a co-precipitation method, and the resultant catalyst is a mixture of Cu, Zn, and Al oxides. For example, U.S. Pat. No. 4,436,833 discloses a co-precipitation method comprising mixing a solution of Cu, Zn, and Al nitrates with sodium carbonate as a precipitation agent to form a carbonate precipitate, washing off the sodium ions with distilled water, and drying and calcining to produce a mixture of Cu, Zn, and Al oxides for catalytic synthesis of methanol. The catalyst, however, has the disadvantage that it is difficult to wash off the sodium ions and control the temperature during the reduction process, resulting in a significant decrease of the catalytic activities.
U.S. Pat. No. 4,366,260 discloses a method for preparing methanol, or mixtures of methanol and dimethyl ether. The catalyst used in the method is a Raney Cu catalyst which is prepared from an alloy containing from 35 to 60% by weight of aluminum, 0.1 to 25% by weight of zinc, and the remainder being substantially all copper. It is known to those skilled in the art that the alloy of the Raney Cu catalyst is a crystalline alloy. The methanol yield by means of the catalyst under the reaction conditions suitable for preparing methanol from syngas is relative low (10.9% only).
Dimethyl ether (DME) is a widely interesting, environmentally friendly, super-clean replacement product for civilian and automobile fuels. There are mainly two production methods of dimethyl ether, i.e. one-step method and two-steps method. The two-steps method resides in synthesizing methanol from syngas, and then dehydrating to produce dimethyl ether. The one-step method means that dimethyl ether is synthesized from the feedstock syngas in one step, and comprises three main reaction steps which are relevant from each other and proceed sequentially:CO+2H2→CH3OH  (1)2CH3OH→CH3OCH3+H2O  (2)CO+H2O→CO2+H2  (3)
Although all three reactions are reversible, the whole reaction procedure can proceed in a state which deviates from the thermodynamic equilibrium since products from each of the reaction step are consumed in the next reaction. Therefore, compared with a single methanol synthesis reaction, the conditions for the dimethyl ether synthesis reaction process directly from syngas are much milder, and the one-pass CO conversion is much higher. Compared with the two-steps method, the one-step method for dimethyl ether synthesis is carried out without an intermediate procedure for the methanol synthesis, and it has the advantages of simpler procedure, less devices, and lower investment and operation cost. Therefore, the production cost for dimethyl ether is decreased and the economic benefit is increased. Thus, the one-step method for dimethyl ether synthesis is of great interest in the research & development in many countries. The catalyst system for the one-step synthesis of dimethyl ether is generally a physical mixture of a methanol synthesis catalyst and a methanol dehydration catalyst. The industrial catalyst for methanol synthesis generally contains one or more of Cu, Zn, Al and Cr, and it is well known for those skilled in the art that the catalyst is a crystalline alloy, whereas the methanol dehydration catalyst is generally chosen from solid acidic materials.
U.S. Pat. No. 5,389,689 discloses a preparation method of the catalyst for producing dimethyl ether in one-step, comprising pulverizing the mixture containing zinc oxide, copper oxide or chromic oxide, and aluminum oxide to particle sizes of from about 0.1 to 20 μm, pressing under a pressure of 100-500 kg/cm3 to adhere the oxides together, then suspending in the solvent and again pulverizing the slurry formed therefrom to obtain the catalyst. Under the reaction conditions comprising a H2/CO molar ratio of 1, a reaction temperature of 280° C., and a reaction pressure of 3 MPa, the CO conversion is 60.1%, the yield of dimethyl ether is 42.8%, and the yield of CO2 is 14.4%. The catalyst for producing dimethyl ether has a low activity, the temperature desired is relatively high, and the CO conversion is relatively low. Moreover, about one third of CO is converted into useless CO2 due to the low hydrogenation activity of the catalyst. Other side reactions occur during the reaction procedure, resulting in carbon availability of generally less than 60%. The reaction process is less cost-effective.
Low carbon olefins (light olefins) which mainly refer to ethylene and propylene are very important raw materials for the chemical engineering. Presently, more than 90% of low carbon olefins are produced from the cracking of light oils. The supply of low carbon olefins is unable to meet the demand in the market. In view of the economic continuous development, and increasing deficiency of the petroleum source, it is absolutely necessary to produce low carbon olefins from a replacement source. The technique for directly preparing olefins from syngas originates from the traditional F-T synthesis. Since the carbon number of the product obtained from the F-T synthesis catalyst follows the S-T distribution law, the selectivity of low carbon olefins is low. The preparation of catalysts having a high activity and selectivity is of great interest in the current research field.
CN1065026A discloses a catalyst for preparing ethylene from syngas. The catalyst comprises one oxide of the element selected from Si, Al, Ti, Nb, and Hf, one or two oxides of the element selected from Nb, Y, Ga, Ge, In, and TI, one or more oxides of the element selected from Sn, Pb, Sc, La, Pr, Ce, and Nd, and is prepared by a method selected from impregnation, co-precipitation, mechanical mixing, slurry-mixing, a combination of impregnation and co-precipitation, or a combination of mechanical mixing and impregnation. Although the ethylene selectivity can be up to 94% when the catalyst is used in the preparation of ethylene from syngas, the CO conversion is only 15%.
CN1390640A discloses a nano copper catalyst for synthesizing methanol or dimethyl ether, and preparation method thereof. The catalyst consists of metal Cu, ZnO, alumina, etc., wherein the molar ratio among these components is 30-60% Cu:60-30% Zn:10-15% Al. The nano Cu has a size of from 2 to 10 nm and is highly dispersed on a complex carrier of ZnO-alumina. The catalyst is prepared by forming a co-precipitate slurry containing Cu, Zn, and Al compounds by a co-precipitation method; and then directly reacting the co-precipitate slurry with a reducing agent to reduce Cu; the reducing agent is remained in the solution after being oxidized by Cu2+, and the Cu2+ compound aggregate in a nanometer size is reduced and then the resultant crystalline elemental Cu in a nanometer size is dispersed on the carrier; then the precipitate is filtered by suction, washed, and dried under vacuum; finally the product is tableted, or calcined under an inertia gas or vacuum at 250-400° C. to obtain the catalyst. However, the yield of methanol in the methanol or dimethyl ether synthesis process using the catalyst is relatively low. For example, the yield of methanol at 240° C. is only 6.5%. Moreover, the catalyst has to be subjected to a complex and time-consuming pre-reduction process before being used in the synthesis process.